1. Field of the Invention
The present invention relates to an electronic device of a source driver in an LCD device, and more particularly, to an electronic device for enhancing the accuracy of output voltage to an equivalent capacitor of a panel of the LCD device.
2. Description of the Prior Art
A prior art source driver in an LCD device includes a charge sharing circuit and a precharge circuit, for charge reuse and decreasing a charge time to an expected voltage level for an equivalent capacitor of a panel of the LCD device. Generally, the charge sharing circuit and the precharge circuit are composed of a plurality of transistor switches. A transistor switch is equivalent to a resistor when turned on. The equivalent resistor results in the current limiting effect and inaccuracy of output voltage to the panel and influences the charge time to the expected voltage level for the equivalent capacitor of the panel. To eliminate the equivalent resistor effect, the size and area of the transistor switch has to be enlarged. Therefore, in design of a source driver, the size of the transistor switch used in the charge sharing circuit and the precharge circuit need to be customized for an acceptable equivalent resistor.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a block diagram of an electronic device 10 of a source driver in an LCD device according to the prior art. The electronic device 10 is utilized in the source driver for charge sharing and precharging for a channel of the source driver. The electronic device 10 comprises an output unit 100, a node VCH, and switches 102, 104, and 106. The output unit 100 coupled to a capacitor 108 comprises a switch 101, for generating an analog signal and outputting the analog signal by controlling the switch 101 according to a control signal S1 generated by the source driver. The capacitor 108 is an equivalent capacitor as a characteristic model of a panel of the LCD device. One terminal of each of the switches 102, 104 and 106 is coupled to the output unit 100 and the capacitor 108; the other terminal of each of the switches 102, 104 and 106 is respectively coupled to the node VCH, a first voltage generator 14 and a second voltage generator 16. The switches 102, 104, and 106 control the connections between the two terminals according to the control signals S2, SA and SB. In addition, the first voltage generator 14 and the second voltage generator 16 are utilized for generating voltages with opposite polarity.
When the output unit 100 outputs the analog signal according to the control signal S1, the control signals S2, SA and SB respectively turn off the switches 102, 104 and 106, thereby the output unit 100 outputs the analog signal to the capacitor 108. On the contrary, when the output unit 100 does not output the analog signal, the control signals S2, SA and SB respectively control the switches 102, 104 and 106 for charge sharing and precharging. In detail, when the control signal S2 controls the switch 102 to be turned on and the control signals SA, SB control the switches 104, 106 to be turned off, the source driver performs charge sharing on the capacitor 108. When the control signal S2 controls the switch 102 to be turned off, and the control signals SA, SB control the switches 104, 106 to be turned on, the source driver performs precharging on the capacitor 108.
Note that, the electronic device 10 is utilized for a single channel of the source driver. For charge sharing of a single channel, the capacitor 108 transmits charge to the node VCH by the switch 102. The charge sharing scheme is identical on each channel of the source driver. At the same time, there is a current limiting resistor on the current path of charge sharing. The current limiting resistor is also an ON-resistance of the switch 102. Let the value of the ON-resistance of the switch 102, 104 or 106 be R, so that the value of the current limiting resistor is R. In addition, the switches 104 and 106 are turned on in turn when the source driver performs precharging on the capacitor 108, thus, for each channel, the current limiting resistor on the current path of charge sharing is the ON-resistance of the switch 104 or 106, R.
For the relationship between the voltage level of the capacitor 108 and the control signals S1, S2, SA and SB, please refer FIG. 2. FIG. 2 is a timing diagram of the electronic device 10. CH1 and CH3 represent the output of odd channels of the source driver. CH2 and CH4 represent the output of even channels of the source driver. Note that, the connection between the switch 104 and the first voltage generator 14 (or between the switch 106 and the second voltage generator 16) is different in odd channels and in even channels. Therefore, the source driver can precharge on the capacitor 108 for opposite voltage level by odd channels and even channels.
Because the switch 102, 104 or 106 are transistor switches and the ON-resistance of the switch 102, 104 or 106 is also a current limiting resistor, the current limiting resistor results in inaccuracy of output voltage to the capacitor 108 and influences the charge time to an expected voltage level for the capacitor 108. In this situation, the only way to enhance output voltage accuracy is to adjust the size of the transistor switch. Due to a large amount of transistor switches used for charge sharing and precharging in the prior art source driver, the area and production cost of the charge sharing circuit and the precharge circuit are expensive.